fix: always copy template resources before build

[yufeih]

#8510 uses an unreliable way of fixing #8500 by ignoring files already on disk. It causes side effects such as template not updating when _site was built previously.

This PR attempts to fix #8500 by copy templates before building resources.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02 🎉

Comparison is base (86c61f7) 76.26% compared to head (d72eb38) 76.29%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8522      +/-   ##
==========================================
+ Coverage   76.26%   76.29%   +0.02%     
==========================================
  Files         858      858              
  Lines       30647    30639       -8     
==========================================
+ Hits        23374    23377       +3     
+ Misses       7273     7262      -11     

Impacted Files	Coverage Δ
...ft.DocAsCode.Build.Engine/SingleDocumentBuilder.cs	82.41% <100.00%> (+0.09%)	⬆️
...ild.Engine/TemplateProcessors/TemplateProcessor.cs	87.87% <100.00%> (-0.13%)	⬇️

... and 6 files with indirect coverage changes

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